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Form a group with a tank, healer, and dps. Gather up 15 Sra'thik. AoE them to death within 5 seconds of each other.

Wait for a day when Niuzao is rampaging around the courtyard. Watch him so you get an idea of how fast he moves. (This will help you time your own actions.) Find an area in the courtyard with at least 15 Sra'thik that you can gather up. Wait for Niuzao to reach the right spot, then aggro and tap at least 15 Sra'thik. Any "uh-oh" survival abilities/spells ought to be used at this point, staggering them if need be. Run into the ox's path and watch him demolish them.

Now, going by the name of the achievement, it's obvious Blizzard designed the achievement to be completed by the latter means.

Scouted around the courtyard from the air. I looked for a dense gathering of mobs within 40 yards of each other. Why? Because Battle Horn makes this easy mode. Still, if you haven't farmed the Battle Horn yet then I recommend having a speed boost potion so you can quickly gather the mobs up while also staying outside of their melee attack. Their melee attacks hit fast and can apply a bleeding dot. Multiply that by 15.Ouch.

Found a spot to my liking at the outer eastern-ish (going by the zone map) corner of the temple.

Took a minute to observe Niuzao's pace. I decided to land on the ground when the ox was solidly on the outer ring of the courtyard.

Landed, cast Ice Barrier, then cast Battle Horn's use effect. I waited until most of the pull was in range, then cast Arcane Explosion two or three times - remember, the mobs must be tagged or no credit is given, and Battle Horn only gives you aggro - then cast Blazing Speed and ran in Niuzao's direction to get ahead of the pack. At this point, my health was down to approximately 55% because I had eight or nine dots ticking on me.